The screw on the top adjusts the pressure, but if it is fluctuating, then the valve is most likely defective.

: : The safe pressure for a boiler, assuming it is a hot water system, is whatever it takes to lift the water to the highest radiator plus about 5 pounds as a fudge factor. It should never go over 30 psi unless the boiler is rated at a higher pressure. If the system needs 25#, for example, and the pressure drops to 10 or 15, then the radiators on the highest elevations will not heat.

: : : Got a quick question. What's a safe pressure for a boiler,and what is the affect if the pressure is too low,say 10 or 15 lbs,instead of 25 -30. Curious.

: I was going to add something but you got back to me too fast. How do you adjust the valve for a higher setting,in or out?